Re: [PATCH v3 1/5] add metadata_incore ioctl in vfs

[Boaz Harrosh <bharrosh-C4P08NqkoRlBDgjK7y7TUQ@public.gmane.org>]

On 01/20/2011 08:27 AM, Andrew Morton wrote:
> 
> Another way of doing all this would be to implement some sort of
> lookaside cache at the vfs->block boundary.  At boot time, load that
> cache up with all the disk blocks which we know the boot will need (a
> single ascending pass across the disk) and then when the vfs/fs goes to
> read a disk block take a peek in that cache first and if it's a hit,
> either steal the page or memcpy it.
> 

Ha. this sounds very much like the cleancache project presented for
inclusion so many times. It has even visited and left linux-next
a few times. They solved all these problems with a few VFS hooks.

> It has the obvious coherence problems which would be pretty simple to
> solve by hooking into the block write path as well.  

See cleancache they solved it with a simple VFS hook.

> The list of needed
> blocks can be very simply generated with existing blktrace
> infrastructure.  It does add permanent runtime overhead - once the
> cache is invalidated and disabled, every IO operation would incur a
> test-n-not-taken-branch.  Maybe not too bad.
> 
> Need to handle small-memory systems somehow, where the cache simply
> ooms the machine or becomes ineffective because it's causing eviction
> elsewhere.
> 
> It could perhaps all be implemented as an md or dm driver.
> 
> Or even as an IO scheduler.  I say this because IO schedulers can be
> replaced on-the-fly, so the caching layer can be unloaded from the
> stack once it is finished with.

Or a cleancache driver

Boaz